In certain beliefs, karma is regarded as the soul migration of a deceased into the body of a new born being.
I would like to say: You do the karma yourself. That means: As we have behaved and learned today in a positive or negative sense - saved as midpoints - we practically shape ourselves for future similar situations for which this behaviour fits. It can be seen as a migration of our past behaviour into present and future behaviour - naturally in the same life. After death, not only the body disintegrates, but also the psyche, which is viewed as a spiritual (spiritual) concept in various beliefs.
As we have behaved in the past, structures have emerged that can be reactivated in similar situations. Life uses this very often. That means that our current behaviour shapes that of the future.
It is the goal-and-way topic: It is part of life that new goals are constantly being formed, to which you have to go ways and form. If there are similarities with earlier shapes and ways, then these are stimulated again because this is an economical principle for the brain. This is rebirth, positive or negative karma.
The past is thus reborn in this sense.
Who e.g. regularly alleviates his stress with alcohol, drugs, smoking etc. (unconsciously through the midpoint mechanics), this behaviour is repeatedly forced with feelings in similar situations to use this again (as a “solution”).